A water-absorbent resin has been widely utilized in various applications such as hygiene articles including disposable diapers, sanitary napkins, incontinent pads for adults and the like; and a water-retaining agent for soil, due to its absorbing property of such a large quantity of aqueous liquid as several times to several-hundred times own weight, and has been produced and consumed in a large quantity. Such a water-absorbent resin (it may also be called a high water-absorbent resin or a water-absorbing polymer) is described, for example, in Japanese Industrial Standard (JIS) K7223-1996, as well as introduced in many commercial reference books.
Although a sheet, a film, a fiber, gel, emulsion or the like have also been known as a form of the water-absorbent resin, a particulate is general, and it is provided in a powder state by obtaining a hydrogel-like polymer by polymerization of a hydrophilic unsaturated monomer, and drying it. The hydrogel-like polymer is mostly obtained as a bulk or aggregated substance of hydrogel particles, which is usually dried so as to give a solid content of about 95% by weight, and then pulverized to a particle with a desired size using a pulverization machine. Because the pulverized substance generally has a certain particle size distribution, however, particles having a particle diameter outside a desired range also generate. Therefore, by sieve classification of this pulverized substance after drying using a classification machine, particles having a size within a desired particle diameter range are prepared. As a result, a particulate water-absorbent resin is obtained. As the particulate water-absorbent resin to be used in hygiene articles, although different depending on intended applications, particles having a particle diameter in the range of usually 106 m or larger, still more 150 μm or larger and below 850 μm, as a major component, are preferably used. On the other hand, fine powder with a particle diameter below 106 μm, still mote below 150 μm, would blow up as powder dust and deteriorate work environment, in fabrication to hygiene articles, as well as causes deterioration of liquid permeability of the water-absorbent resin, and thus it is desirable that amount of fine powder contained in the water-absorbent resin is as low as possible. In addition, particles having a particle diameter of 850 μm or larger would provide convex-concave feeling at apart contacting with a skin, or decrease absorption rate, when used as a material of a hygiene article, and thus it is desirable that particles having a particle diameter of 850 μm or larger contained in the water-absorbent resin, are as less as possible, and usually particles having a particle diameter of 850 μm or larger are classified by sieve classification and then re-pulverized. Load of the pulverization machine or generation of fine powder, in this case, has also been a problem (refer to PATENT LITERATURES 1 to 4).
As described above, a water-absorbent resin is preferably in a form of particles having the upper limit and the lower limit of particle diameter controlled, and as a control method of such a particulate water-absorbent resin, there have been known a method for controlling particle diameter in a polymerization step such as reversed phase suspension polymerization (refer to PATENT LITERATURES 5 and 6); a method for controlling in a fine granulation step of a hydrogel-like polymer (refer to PATENT LITERATURES 7 and 8); a method for controlling in a pulverization step of a dried substance (refer to PATENT LITERATURES 9 and 10); a method for controlling in a classification step (refer to PATENT LITERATURES 11 to 16); a method for granulating in surface cross-linking (refer to PATENT LITERATURE 17); a method for removing fine powder with air flow in a cooling step after surface cross-linking (refer to PATENT LITERATURE 18); a method for making a large particle by adhering particles themselves after surface cross-linking (refer to PATENT LITERATURES 18 to 22), or the like.
However, control of a particle diameter in polymerization disclosed in PATENT LITERATURES 5 and 6 has limitation, as well as had still more difficulty in controlling a particle diameter in aqueous solution polymerization, unlike in the case of reversed phase suspension polymerization where polymerization is performed in a particle state.
In addition, also in a method disclosed in PATENT LITERATURES 7 to 10, there is limitation in controlling a particle diameter, and effect thereby is insufficient only by improvement of a pulverization apparatus, which thus requires search for conditions of a dried substance which permits efficient pulverization. A method for pulverizing a water-absorbent resin with high water content generally requires higher load on a pulverization machine, as compared with pulverization of a water-absorbent resin with low water content, and still more limits a pulverization method and a pulverization machine which can be used. As disclosed in PATENT LITERATURE 10, in a method for performing forced cooling, it is possible to suppress generation of fine powder in pulverization or particles which cannot be pulverized to a desired particle diameter, even by passing through a pulverization machine, as long as temperature of a dried substance is near room temperature. There, however, is a problem of requiring large scale facility, in using cool air or increasing air amount, due to inferior cooling efficiency in performing further cooling from 50° C. using air at room temperature.
Further, although removal of fine powder by classification disclosed in PATENT LITERATURES 11 to 16 and PATENT LITERATURE 18 enabled to remove fine powder, in some cases, it required not only disposal or reuse (recycle) of a large quantity of fine powder, but also classification operation for a long period for removing only fine powder completely. In addition, a granulation method disclosed in PATENT LITERATURES 17 to 22 showed a case where fine powder was reproduced in use or during transportation, caused by weak granulation strength, or absorption property was decreased, caused by use of a binder or the like in granulation.
Still more, a recycling method for a water-absorbent resin fine powder in a producing step described in PATENT LITERATURES 23 to 26 has a problem of requiring facility to recover and process fine powder, as well as requiring operation cost therefor, and thus it is necessary to reduce generation amount itself of fine powder. In addition, a method for recycling fine powder to a monomer of a polymerization step or a hydrogel-like polymer, described in PATENT LITERATURES 24 to 26 and the like, generated, in some cases, a problem of polymerization inhibition caused by fine powder.